1. Field of the Invention
This invention relates to an optical device functioning as a photonic transistor. The invented optical device can be used as a basic element in an optical integrated circuit (OIC) that permits logic operations to be carried out using optical signals.
2. Description of the Related Art
In electronic devices, transistors are the basic elements of circuits. In digital applications, a transistor has the properties of a switch. The transistor can be driven between conductance and non-conductance states in order to change the voltage level, and thereby logic level, output by the transistor.
Interest has begun to emerge in recent years toward development of an optical device that behaves analogously to a electronic transistor. The reason for this interest is that optical signals can potentially travel faster in integrated circuits than electrical signals because they are not subject to capacitance which slows switching speed between logic states. Given the ever increasing demand for faster switching, it is expected that in the future, absent a major technological advance in electronics, use of optical devices will become increasingly desirable if not essential.
However, use of optical devices to form integrated logic circuits presents unique challenges. By its nature light propagates and cannot be stored. The ability to represent a logic state stably for as long as may be required thus becomes an issue. It would thus be desirable to provide a photonic transistor that can be used to represent logic states stably using optical signals. Moreover, there is an established industry using optical components which use primarily amplitude-modulated optical signals in which the amplitude or intensity of light pulses represents digital logic states. Any solution able to store and process data optically will also ideally be compatible with existing optical telecommunications infrastructure.
Also related to this application are light-guiding elements, which include materials such as photonic bandgap (PBG) elements. PBG elements are composed of structures with periodic spacing that enable light of a wavelength related to the spacing of the structures to travel in a confined manner through the material. Although guiding elements are interesting from the standpoint that they automatically filter light of undesired wavelengths, an undesired result is that light is quickly attenuated in guiding elements. It is therefore desirable that optical logic and signal processing be performed with a relatively short transmission pathway through a guiding element such as a PBG material to avoid its being unduly attenuated.
In the manufacture of virtually any integrated circuit, it is desirable that the components of the circuit be integrated on the substrate in a relatively small area to enable the most functionality possible per unit area of the device. Although conductive electrical wires and the like can turn abruptly in connecting to electrical transistors, optical waveguides can not generally turn so abruptly without use of mirrors because excessive light will escape the waveguide. Thus, integrated optical waveguides have limits on how abruptly they can turn which further impacts on the size of the integrated device and creates issues regarding optical isolation of the integrated devices. It would be desirable to provide an integrated device in which optical signals can propagate in abrupt turns to increase functionality of integrated optical devices.
Moreover, as more optical devices and related connections are integrated on a substrate, it becomes desirable to optically isolate the devices to avoid multi-path effects and cross-talk between devices. It would thus be desirable to provide an optical circuit in which the optical devices are effectively optically isolated from one another.
In some optical modulation schemes, data is represented by more than two amplitude levels. The problem with such an approach is that it requires very stringent control on the amplitudes of the optical signals on which logic operations are performed. For example, in an AND gate, if two pulses are both at high or “1” logic levels represented by an amplitude of “1” in this example, then the output will have an amplitude that is the linear sum of these two levels, or “2”. This output signal must be attenuated back to “1” before it can be provided to the next optical gate in the circuit. This approach for an optical modulation scheme is not generally desirable because of the complications associated with maintaining appropriate amplitude levels throughout the circuit. It would therefore be desirable to provide an optical circuit that does not require management of the amplitudes as required in such linearly additive optical circuits.
The publication “Fabrication and Characterization of Photonic Crystal Slab Waveguides and Application to Ultra-Fast All-Optical Switching Devices,” Kiyoshi Asakawa, The Femto Technology Research Association, Jun. 30, 2003, discloses a symmetrical Mach-Zender interferometer formed in a photonic crystal. The input side of the device includes three input paths for light pulses to enter. The outer paths receive a control pulse, and because one of the two outer paths is different in length from the other, the control pulse received by both of these outer paths is delayed in one path by π radians relative to the other path. An optical signal pulse is input to the central path, which branches into two separate paths that meet with respective outer input paths. At the two places where the outer and inner symmetric paths meet, the crystal slab has two quantum dot nonlinear elements, one for each of the two meeting places. The quantum dot nonlinear elements outputs are provided to initially separate pathways that join together to form a single output path for the output light pulse. The Asakawa device represents binary zeros with the lack of light, which cannot be used to perform digital logic, because the device transmits no data if no light enters it. The Asakawa device was thus created with the intention of using it as a switch or filter for incoming fiber optic data. It would be desirable to provide a photonic transistor that can be used as the basis for gates to perform optical digital logic for generic all-purpose optical computing.
Also worth mentioning in relation to this disclosure is US 2003/0179425 filed Jan. 27, 2003 and published Sep. 23, 2003, naming Charles Romaniuk as sole inventor. The application discloses a device with a combiner stage followed by a filter stage, next followed by an output stage. The combiner stage includes two y-shaped combiners, the first of which receives two phase-modulated input signals to generate an output signal that is supplied to one of two inputs to the second combiner. The second input to the second y-shaped combiner is a control input. The filter stage includes an absorption diode which receives the output of the second combiner of the combiner stage, and generates a binary output based on same. The output of the y-combiner of the output stage receives as one input the output of the absorption diode of the filter stage as its input. The combiner of the output stage also receives a second input which is another control signal. Depending upon the state of the two control inputs, which are π radians out of phase from one another, the logic circuit functions as either an AND or OR logic gate.
Although Romaniuk's device is meritorious in several respects, it requires use of both phase and amplitude modulated inputs to perform complex logic operations, and thus requires a linear absorber to discern logic levels. More specifically, in the Romaniuk device, if two signals interfere constructively, the magnitude of the output becomes twice as large. If this interference continues to propagate through a circuit as it does in the Romaniuk device, complications result when it is used as an input to a subsequent logic gate with an amplitude twice as large as it was originally. By absorbing half of the signal, the Romaniuk device can lessen the interference back to 1× amplitude. However, this requires exacting control of the amplitude of the logic levels in the device through the use of a linear optical absorber.
For many applications, it would be desirable to utilize a different approach in which complimentary networks with sets of photonic transistors which are activated or deactivated, depending upon the states of the input signals. This approach can be used to avoid representation of more than two digital logic states with different amplitudes that linearly add, which is highly subject to error without extensive control of amplitude levels throughout the circuit.
Thus, although the published patent application U.S. 2003/0179425 has its merits, it would be desirable if a device could be obtained that is relatively simplified, capable of integration on a substrate, does not require use of linearly additive signals propagating through its circuit requiring complexity to discern and interpret logic levels, and yet one that provides effective logic operations on optical signals.